1. Field of the Invention
The invention relates to a method for vacuum or gauge pressure shaping in a mold of a foil, sheet or film, more particularly of a textile, plastic or textile-like material which can be deformed in cold or heated condition, more particularly in the production of cushions of soft foamed plastic having an integrated skin or cover of sheet material; and to apparatus suitable for carrying out the method.
2. Description of the Prior Art
The production of cushions of elastic foamed plastic having a composite surfacing structure of fabric or the like has previously been proposed, wherein a sheet or film of textile-like material, which can be deformed when cold or in heated condition and made impermeable to air by an elastic coating of one of its surfaces, is laid across the open face of a suitable configured cavity in a molding die, following which the sheet material is stretched while being brought into snug conformance under the action of vacuum (or pressure) with the wall of the molding die. A foamable plastic mixture is then poured into the mold cavity, while lined with the sheet member, and this mixture foams in the cavity and forms the body of a cushion. After polymerisation of the foam material, the suitably configured body of foam with the surfacing permanently attached to the foam material, is removed from the mold cavity.
A substantial difficulty in the case of this known method lies in the achievement of a uniform thickness and extension of the fabric surfacing structure of the finished product. It is to be conceded that on establishing the vacuum in the mold cavity, the whole fabric is more or less evenly stretched as it initially comes into contact with the shallower parts of the mold cavity in the molding die. Those areas of the fabric which contact the wall are then fixed, however, against further stretching by the combined action of the vacuum and the friction against the mold wall.
Where there is substantial depth variation in the mold contour, those areas of the fabric which do not initially contact the mold wall must undergo additional stretching to bring them into contact with the deeper part of the mold cavity. This leads as a result to different degrees of stretch or extension of the fabric in different areas on the surface of the cushion, and therefore to locally different thicknesses of the fabric surfacing structure on the finished cushion. In other words, the frictional forces effective at the initially contacted surface impair even extension or stretch during the final stage of shaping to conform to the deeper areas of the mold cavity. This eventually means that in the case of the finished product (e.g. a cushion) it is the very areas of the cover which are subject to the heaviest loading that are thinnest.
In order to obtain more uniformity of the thickness of the fabric all across the surfacing structure, it becomes necessary to effect a sliding movement of the fabric on the cavity wall of the molding die after initial contact is made. This however will not occur for the above mentioned reasons, and it has been the practice heretofore to apply lubrication of some form to make possible such a sliding movement of the fabric. However this is not very practical because it causes fouling of the cover of the finished cushion. Furthermore clogging of the vacuum exhaust holes in the mold wall presents a definite manufacturing disadvantage.
In order to be able to deal with, or at least to reduce the above mentioned difficulties, it has also been suggested heretofore to use a mold plunger, or a number of such plungers, to mechanically press the fabric into the deeper areas of the cavity of the molding die before the vacuum becomes established in the mold cavity. By the use of such mold plungers the fabric is subjected as a whole to a first, more or less, even deformation to give it a preliminary shape more nearly approaching that of the mold cavity, before the vacuum becomes established; thereafter the fabric is brought into final complete conformity to the die by undergoing an additional deformation of substantially lower order, as effected by application of vacuum at such time.
This manner of operation, which makes possible a shaping in the mold of the fabric under tension which, all-in-all, is relatively even, however requires additional mold devices which are complicated and expensive, especially where the cavity wall of the molding die is vaulted and has shapes which are difficult to produce. Furthermore such additional devices are not practical in the case of molding dies which are continuously advanced, as in a production line involving automated manufacture of the cushions.
There has also been a previous proposal (see German patent application No. P 25,11,811.0) to arrange an elastically deforming perforated membrane of varying thickness (possibly employing supplementally a liquid or solid lubricant) between the fabric and the mold wall. Owing to the variation in thickness, this membrane has a corresponding variation in stretch capacity, the localized membrane thickness being so selected that areas of greater thickness coincide with regions of deeper contouring in the mold cavity. The net effect of this is a relatively even stretch or extension of the fabric, since the fabric facing sheet frictionally adheres to the membrane surface and its stretch is accordingly controlled by the stretching action of the membrane.